Airborne weather radar systems are used to detect the presence of storms and turbulent weather. This is important since weather is a cause or contributing factor in a large number of aircraft accidents. Weather forecasts are useful but airborne weather radar provides invaluable real time input as to local conditions ahead and in the immediate vicinity of the aircraft. Accordingly, weather radar serves as the main focus of the pilot's awareness of weather conditions that may affect the aircraft. However, radar reflectivity at the frequencies employed in weather radar systems is primarily a function of the amount of atmospheric moisture. Modern weather radar is first and foremost a gauge of rainfall providing an excellent graphical depiction of precipitation conditions relative to the aircraft's position. On most weather radar displays different colors are used to represent different radar return signal levels and correspondingly different amounts of moisture detected in the atmosphere. Typically, the color black is used to represent very little or no rainfall; green is used to represent weak rainfall; yellow is used to represent moderate rainfall; and red is used to represent strong rainfall. It's up to the pilot to then interpret the display and to infer the presence of hazardous flight conditions from the image features depicted on the display. It takes training and experience to do this effectively and can represent a substantial workload to pilots who are often busy performing other essential tasks. Consequently, weather radar may not be used to its maximum potential and hazardous conditions may sometimes be missed.
Thunderstorms are a major hazard to aircraft and an understanding some of their characteristics is important to pilots in interpreting weather radar. Thunderstorms are composed of three vertical parts. The bottom portion, below the freezing level, is composed mostly of rain that reflects weather radar signals very well and serves as the most reflective portion of the storm (“bright band”). The middle portion occurs above the freezing level and up to the altitude level or tropopause where the ambient air temperature drops below −40 degrees Celsius. This section of the storm is composed of a combination of ice crystals having low reflectivity and super cooled water providing moderate reflectivity. This middle portion shows up well on weather radar but its radar reflectivity diminishes with increasing altitude up to the −40 degree C. level often referred to as the radar or wet top of the storm. Severe thunderstorms can often be distinguished from more benign storms by vertically scanning the storm and determining the amount of vertical development in the storm above freezing level and the amount of the storm above the tropopause level. Above the −40 degree C. level up to its actual or visible top, a storm is composed entirely of ice crystals that reflect very little radar energy. However this top section above the radar top may be the site of serious turbulence and comprise an area hazardous to aircraft.
Given the right conditions with large amounts of warm and humid air rising from the ground thunderstorms can grow with surprising speed and ascend in altitude as rapidly as 6000 ft. per minute. As warm and humid air is caught in strong updrafts and converted to ice crystals and super-cooled water it may rise up well above the freezing level. Large amounts of moisture can be trapped and held aloft high in a thunderstorm cell forming an area of dangerous potential energy for creating hazardous weather. This phenomenon is referred to as thunderstorm vaulting. During such events, limited precipitation occurs in the bottom portion of the storm as few downdrafts exist and the true extent of the storm may be misjudged unless the middle portion of the storm is scanned and the developing nature of the threat recognized.
U.S. Pat. No. 5,049,886 to Seitz et al describes a weather radar system in which the weather data is converted into polar coordinates and is rotated and translated on the display screen in accordance with turning maneuvers executed by the aircraft. This patent also describes weather radar systems having combined displays of PPI and RHI data.
U.S. Pat. No. 4,940,987 to Frederick describes a radar system featuring an automatic horizontal and vertical scanning radar for displaying weather conditions in combined plan view and vertical view images. The vertical view may comprise a vertical front view display or a vertical side view display having altitude indicia so that the pilot can perceive heights and intensities of storm cells and select the best route through adverse weather conditions.
U.S. Pat. No. 6,650,275 to Kelly et al describes a weather radar system which processes radar images to identify features such as squall lines, hooks and steep reflection gradients that are potentially indicative of hazardous conditions. Notifications may then be provided on the radar screen such as textual information or visual highlighting that warns the pilot about the hazardous conditions.